$ cat file.txt
aaa
b
b
c
c
b
b

I am adding a second field to all lines of file.txt as "1", then using associative array.

$ awk '{print $0,1}' file.txt | awk '{arr[$1]+=$2} END {for (i in arr) {print i,arr[i]}}'
aaa 1
b 4
c 2